The IEEE standard IEEE 802.1D defines transparent bridges which use spanning tree protocol (STP) or rapid spanning tree protocol (RSTP) to avoid loops by limiting the usable topology of a physical topology to a mathematical spanning tree. The spanning tree protocol determines a root node and establishes a spanning tree which is optimal with regards to path cost to the root node. A path cost is a configurable numerical parameter given to a bridge port. The path cost, in most cases, is based on a guideline given in various versions of IEEE802.1D and IEEE802.1Q standards. The path cost can be administratively set per port. Another parameter of a path is a root path cost. The root path cost is a value that bridges exchange using Bridge Protocol Data Units (BPDU). In general, each bridge uses the root path cost on each port to determine how far the root bridge is away on this port. The following steps are carried out in order to establish the root path cost. Each bridge sends out its own root path cost. The root starts with 0. Furthermore, each bridge chooses the port with the lowest received root path cost as the root port. The root port calculates its own root path cost by adding the path cost of the port where it received the BPDUs to the received root path cost. Then this bridge sends BPDUs with its own root path cost. Furthermore, this standard defines default values for various Ethernet link speeds. Multiple spanning tree protocol (MSTP) that is defined in IEEE802.1Q standard is an enhancement to RSTP, which provides multiple instances of the spanning tree on the same physical topology. STP, RSTP, and MSTP rely on the exchange of messages called Bridge Protocol Data Units (BPDU) to form the spanning trees and to change the spanning trees when necessary. Bridges are either interconnected using wired links or wireless links.
In contrast to wire-line links, wireless links are faced with a time varying transmission path; i.e., under good propagation conditions a high data rate may be transmitted, while under bad propagation conditions only a lower data rate may be transmitted. Solutions known in the art use a fixed data rate adapted to the desired availability of the link. It is worth to note that propagation conditions are good most of the time. However, the requirement of availability close to 100% leads to a choice of a very robust modulation and coding scheme. In this robust modulation and coding scheme the availability requirement of the link is satisfied, but at the cost of reduced data speed of the link. Therefore, the achievable data rate is small compared to what could have been achieved most of the time.
Nowadays, wireless links may employ adaptive modulation techniques. Adaptive modulation, used in digital wireless communications systems, allows the transmitter to adapt its transmission mode in accordance with the condition of the channel. Both directions of a wireless link are independent. The receiver measures the quality of the link (by signal to noise ratio, SNR). If the receiver detects that the SNR has a value which is either too bad or too good for the current PHY mode (i.e. modulation scheme and the coding scheme used on that channel), it sends a message to the sender and requests the sender to change the PHY mode. Depending on propagation conditions of the channel the transmitter may change the modulation scheme and the coding scheme used on that channel. Different order modulations allow sending more bits per symbol and thus achieve higher throughputs or better spectral efficiencies. With the increase of the range, modulations of lower order must be used and conversely with the distance between the transmitter and the receiver getting shorter, higher order modulations for increased throughput can be used. In addition, adaptive modulation allows the system to overcome fading and other interference. The adaptive modulation of a link allows for dynamically switching between various PHY modes. A PHY mode consists of a modulation scheme and a coding scheme and determines the speed of the link as well as its robustness. This allows the maintenance of the desired availability of the link (e.g. 99.99% of time) with the most robust PHY mode, while providing a higher data rate otherwise (e.g. 99.9% of the time).
However a disadvantage of this solution is that when adaptive modulation changes the PHY mode of a link and subsequently its data rate, the existing topology may no longer represent an optimal spanning tree with regards to the usage of available resources. This may lead to wasteful situations where a network actually carries less traffic than it theoretically could do.
When adaptive modulation uses a less robust modulation scheme, errors may corrupt data frames and subsequently BPDUs. If a bridge port is in state “discarding” and in port role “AlternatePort” or “BackupPort” as specified in IEEE 802.1D for at least one spanning tree instance and if this port does not receive BPDUs from a neighbour bridge for a specific period of time the bridge may conclude that this port is connected only to end systems. Subsequently, it may set the port state to “forwarding” causing a loop which leads to catastrophic behaviour. In other cases a bridge might reconfigure the spanning tree to a less optimum topology.
Hence, an improved network bridge and a method of its operation would be advantageous and in particular one that supports dynamic changes of the topology when adaptive modulation changes the PHY mode.